Add Support for Auto-tracking of Multi-sig BTC Wallets

When importing an HD wallet for BTC (and other currencies that give you a new address on every receipt), it is easy to add an xpub key of the wallet so that Koinly can easily track every transaction in the wallet without manually adding them. It would be really great if this same functionality could be added for multisignature wallets as well, because there is no single xpub for a multisignature wallet so each of these addresses must be added manually, which is a pain (especially with change addresses, etc)

This would only require implementing the ability to take a list of xpubs and an m-of-n value and generate the list of addresses (similar to what is done for an HD wallet import). It is a very well defined solution and it should be fairly straightforward, but would make multisig wallets infinitely easier to keep track of in Koinly. The code for multisig address generation is implemented in plenty of open-source watch-only multisig wallets and could probably be repurposed relatively easily. Thanks!